docs: def try_greyscale(pixels, alpha=False, dirty_alpha=True):
      """
      Check if flatboxed RGB `pixels` could be converted to greyscale
  
      If could - return iterator with greyscale pixels,
      otherwise return `False` constant
      """
      planes = 3 + bool(alpha)
      res = list()
      apix = list()
      for row in pixels:
          green = row[1::planes]
          if alpha:
              apix.append(row[4:planes])
          if (green != row[0::planes] or green != row[2::planes]):
              return False
          else:
              res.append(green)
      if alpha:
          return MergedPlanes(res, 1, apix, 1)
      else:
          return res